GEOLOGY AND GEOCHRONOLOGY OF THE SOUTH MOUNTAINS, CENTRAL ARIZONA

Abstract

The South Mountains are composed of two fundamentally different terranes. The western half of the range consists of Precambrian metamorphic and granitic rocks, whereas the eastern half is dominated by a composite middle Tertiary pluton. North-northwest-trending, middle Tertiary dikes have extensively intruded both terranes. A major episode of middle Proterozoic metamorphism and deformation produced a steep crystalloblastic foliation that generally strikes northeast. Middle Tertiary plutonism was accompanied by intense mylonitization that affected Precambrian and middle Tertiary rocks alike. Discrete phases of mylonitization were associated with each intrusive pulse between 28 and 25 m.y.B.P. Mylonitization generally produced a lowangle foliation and east-northeast-trending lineation. The attitude of mylonitic foliation defines a broad, east-northeast-trending anticline that controls the topographic axis of the range. Structurally low rocks in the core of the anticline are nonmylonitic, but intensity of mylonitic fabric increases progressively toward higher structural levels. Mylonitic Tertiary plutonic rocks are exposed as a gently dipping carapace overlying their less deformed equivalents. Mylonitic fabric cuts through the Precambrian terrane as a broad, west-dipping zone. Rocks above and below this mylonitic zone are lithologically identical and mostly retain their Precambrian structure. Fabrics in all rock types indicate that mylonitization resulted from extension parallel to east-northeast-trending lineation and flattening perpendicular to subhorizontal foliation. Mylonitization occurred under conditions of elevated temperature but relatively low confining pressure. Gold-bearing quartz veins occur in tension fractures that are late- to post-kinematic with respect to mylonitic deformation. Mylonitization was succeeded by more brittle deformation that produced chloritic breccia and microbreccia in the footwall of a major detachment fault that dips gently to the east. The detachment fault and underlying breccia were formed by normal faulting and brittle extension in an east-northeast direction. Rocks above and immediately below the detachment fault were antithetically rotated during faulting. Mylonitization, detachment faulting, and formation of the main east-northeast-trending anticline are all manifestations of eastnortheast-directed, middle Tertiary extension. Evidence for a possible continuum between mylonitization and detachment faulting has important implications regarding the evolution of Cordilleran metamorphic core complexes.